The present invention relates to a capsule endoscope which is introduced into a body cavity of a patient for picking up images of the inside of the body cavity.
In recent years, a capsule endoscope system, including a capsule endoscope (an endoscope in the shape of a small capsule) which is swallowed by a patient to be introduced into the patient's body cavity for picking up images of the inside of the body cavity and a processor and a monitor which are placed outside the patient's body, is being developed in order to eliminate the pain of patients in orally introducing (swallowing) the tip of a conventional electronic endoscope formed as a flexible tube.
The capsule endoscope swallowed (orally introduced into a body cavity) by the patient picks up an image of the inside of the body cavity, converts the image into an image signal, and wirelessly transmits the image signal to the processor placed outside the patient's body. The processor receives and processes the image signal and thereby displays the image of the inside of the body cavity on the monitor. Since such a capsule endoscope only requires patients to swallow a small capsule, observation of the inside of the alimentary canal, etc. can be carried out without causing pain to the patients.
FIG. 9 is a schematic diagram showing a capsule endoscope which is employed for a capsule endoscope system. As shown in FIG. 9, the capsule endoscope 100 is enclosed and sealed up by a casing in the shape of a capsule. The capsule endoscope 100 is mainly composed of an objective optical system 101, an image sensor 102 for picking up an image of the inside of a body cavity through the objective optical system 101 and converting the image into an image signal, a signal processing circuit 103 for processing the image signal outputted by the image sensor 102, a transmitter 104 for transmitting the image signal processed by the signal processing circuit 103 to a processor which is placed outside the patient's body, a battery 105 for supplying electromotive force to each component of the capsule endoscope 100, and a lighting unit 106 for illuminating the inside of the body cavity (image pickup range).
The capsule endoscope 100 swallowed (introduced into the body cavity) by the patient is powered by the battery 105. By the capsule endoscope 100, an image of the inside of the body cavity is captured by the image sensor 102, an image signal representing the image is obtained by the signal processing circuit 103, and the image signal is transmitted to the processor by the transmitter 104.
However, with such a capsule endoscope 100 being introduced into a body cavity, it is very difficult to control the direction of the objective optical system 101 (that is, to control the attitude of the capsule endoscope 100). Even if the attitude control of the capsule endoscope 100 is made possible, in order to capture an image of an organ having a large interior wall area (stomach, etc.) by use of the capsule endoscope 100, the image pickup range of the capsule endoscope 100 has to be shifted bit by bit by changing its attitude while capturing a plurality of images and that takes a very long time. Therefore, employment of an objective optical system having a wider field of view as the objective optical system 101 of the capsule endoscope 100 is being hoped for in order to realize more efficient observation.
As an objective optical system having a wide field of view, there exists the so-called omnidirectional image pickup optical system (omnidirectional lateral view optical system) having a field of view of 360 degrees (omnidirectional) around the optical axis of the object lens and being mainly employed as the objective optical system of a monitoring camera (see Japanese Patent Provisional Publication No.2000-131737, for example). FIG. 10 is a schematic diagram showing an example of the application of such an omnidirectional lateral view optical system to an objective optical system of a capsule endoscope. The omnidirectional lateral view optical system includes an object lens 201 and a convex reflecting mirror 210 in the shape of a paraboloid of revolution which is placed in front of the object lens 201, by which an omnidirectional image can be formed on an image pickup plane through the object lens 201. As shown in FIG. 10, the convex reflecting mirror 210 is placed so that its central axis will be coaxial with the optical axis of the object lens 201. Object light (light reflected by the object) within an image pickup range a is reflected by the convex reflecting mirror 210 toward the object lens 201 and is focused on the image pickup plane of an image pickup sensor (photoreceptor) 202 by the object lens 201.
By employing such an omnidirectional image pickup optical system as the objective optical system of a capsule endoscope, an image pickup device having a wide field of view can be realized, by which a wide range inside a body cavity can be observed efficiently regardless of the attitude of the capsule endoscope.
However, even though the aforementioned omnidirectional lateral view optical system is originally designed to be applicable to indoor shooting, capturing images inside a body cavity (with almost no light reaching the object in comparison with indoor shooting with a certain amount of light) by use of such an omnidirectional lateral view optical system is almost impossible. Even if a lighting unit employed for image pickup devices in conventional endoscopes (illumination by an optical fiber, an LED, etc.) is applied to a capsule endoscope having the omnidirectional lateral view optical system, resultant observable range is limited to a narrow range due to the difference between the illumination range of the lighting unit (optical axis direction of the object lens 201) and the image pickup range of the omnidirectional lateral view optical system (all directions orthogonal to the optical axis of the object lens 201).